feat(mail): resizable columns, account in folder tree, mail settings in CRM settings

- Add ResizablePanel component (components/ui/ResizablePanel.tsx) with
  drag-to-resize via mouse events, no external dependencies
- Replace fixed-width columns in Mail.tsx with ResizablePanel for all
  three panes (folder tree, mail list, mail detail)
- Remove account-select toolbar item from Mail.tsx; account selection
  is now integrated into the folder tree
- Restructure MailFolderTree to show mail accounts as top-level nodes
  with folders nested underneath, built from flat list via parent_id
- Add expand/collapse chevrons for accounts and folders with children
- Load folders for ALL accounts (not just selected one) so multi-account
  tree works
- Derive selectedAccountId from selected folder instead of separate
  account selector
- Add Mail tab to CRM Settings page (Settings.tsx) with 📧 icon
- Add /settings/mail route pointing to MailSettingsPage
- Update settings link in mail toolbar to point to /settings/mail
- Update no-accounts empty state link to /settings/mail

frontend/src/pages/Settings.tsx

@@ -14,6 +14,7 @@ export function SettingsPage() {
     { to: '/settings/currencies', label: t('currencies.title'), icon: '\ud83d\udcb0' },
     { to: '/settings/taxes', label: t('taxes.title'), icon: '\ud83d\udccb' },
     { to: '/settings/sequences', label: t('sequences.title'), icon: '\ud83d\udd22' },
+    { to: '/settings/mail', label: t('mail.settings'), icon: '\ud83d\udce7' },
   ];
 
   return (